Fig. 4. Colloidal molecules self-assembled by specific directional bonding between particles. Bright-field optical microscopy (left panels), confocal fluorescent (middle panels A–E) and schematic images (right panels). Scale bars correspond to 2 μm. (A) Complementary green and red monovalent particles form dumbbell-shaped AB-type molecules. Supracolloidal molecules AB₂, AB₃ and AB₄ are formed by mixing red monovalent with green divalent (B), trivalent (C) and tetravalent (D) particles. (E) If complementary divalent particles are mixed, linear alternating polymer chains spontaneously assemble. (F) When particles with bigger patches are used, ‘cis’ and ‘trans’ isomers can form. Introducing more monovalent particles leads to colloidal ‘ethylene’ molecules. Scale bars correspond to 2 μm. (G) Scanning electron microscopy (SEM) image of trivalent patchy particle. Scale bar corresponds to 500 nm. Reproduced with permission from ref. 42.